1. Field of the Invention
This invention relates to a flash photography apparatus in which object distance is detected by pre-flashing and an aperture value is determined appositely to the detected object distance having the aperture of a stop controlled depending on to the aperture value.
2. Description of the Prior Art
In the above conventional flash photography apparatus, as disclosed in, for example, U.S. patent application Ser. Nos. 444,214, 447,508 and 452,875, reflected light quantity received from the object to be photographed as a result of pre-flashing is integrated by means of an integration circuit. Then, the integrated value of the received light quantity is converted into a digital value indicating object distance in a digital value and aperture value is determined based on the digital value which can be expressed as a function of the object distance.
The varying rate of the received light quantity resulting from pre-flashing relative to variation in object distance is such that it varies two-fold every time the object distance varies by .sqroot.2 times. Therefore, arrangements to detect different object distances over a wide range would result in extremely great variations in the output of the above integration circuit. Then, an A-D converter, which is required to convert the output of the integration circuit into a digital value, must have a wide dynamic range for the input thereof in order to obtain a sufficiently high degree of resolution. Such an A-D converter would inevitably necessitate an increase in size. Meanwhile, the flash photography apparatus generally has a limited space, which prevents a large A-D converter from being incorporated therein. As a result, the A-D conversion of object distance values has a very low degree of resolution roughly dividing the object distance range into far, medium and close distances. Therefore, the aperture values that are the determined according to the object distance have only been designated in three steps.
It is therefore a general object of the present invention to solve the above problem of the prior art. A more specific object of the invention is to provide a flash photography apparatus of the above kind, wherein: The output of an integration circuit, which integrates reflected light quantity received from an object to be photographed as a result of pre-flashing, is A-D (analog-to-digital) converted. Then, when the A-D converted value or integrated value is above a predetermined value, the output of the integration circuit is varied and is again A-D converted by the A-D converter. An aperture value suitable for flash photography is designated based on the second A-D conversion value and also a signal indicating of that the result of the first A-D conversion or the original integrated value has exceeded the predetermined value. The resolution of the apparatus is thus enhanced, permitting selection of one aperture value from many designatable aperture values for a wide range of object distances employed with a relatively small, low resolution A-D converter.
Another object of the invention is to provide a flash photography apparatus of the above kind, wherein: The integration output of an integration circuit produced during a preliminary distance measuring process is detected by detecting means which produces a detection output when the integration output reaches a predetermined level. Level reducing means which reduces the integration output, depending on the detection output, transmits the reduced integration output to an A-D converter. An aperture value is determined and designated based on the A-D converted value of the integration output obtained at the end of the preliminary distance measuring process when the integration output does not reach the predetermined level. However, the aperture value is determined and designated based on the A-D converted value of the integration output which is obtained at the end of the preliminary distance measurement and also the detection output when the detection output is produced during the preliminary distance measuring process. Therefore, an aperture value can be selected from many aperture values designatably stepped over a wide range depending on the object distance even when the A-D converter has a low dynamic level value.
A further object of the invention is to provide a flash photography apparatus of the above kind, wherein: When the integration output of an integration circuit reaches a predetermined level during an integrating action on received reflected light resulting from pre-flashing, the output of the integration circuit is reduced at that time with the integration circuit gain shifted (increased) by, for example, shifting the integrating capacity of the integration circuit. Then, information on the gain shift is stored. The above integrating action is further performed from the reduced integration output. The aperture is solely determined by the A-D converted value of the integration output obtained at the end of the integrating action when the above gain shifting is not performed before the end of the integrating action. It is determined based on both the A-D converted value of the integration output obtained at the end of the integrating action and the above gain shift information when the gain shifting action is performed before the end of the integrating action. This arrangement permits enhancement of the resolution of the A-D converter over a wide range of object distances, so that aperture values can be designated for a wide range of object distances even if the dynamic range of the A-D converter in use is low.
It is still a further object of the invention to provide a flash photography apparatus in which: During the process of pre-flashing, when the integrated value of the reflected light quantity received from an object to be photographed reaches a predetermined value, a detection output is produced. Then, the above integrated value is reduced in response to the detection output. The integrating action on the reflected light quantity is then further performed from that reduced. An aperture value is determined solely on the basis of the integrated value of the reflected light quantity if the detection output is not produced during the pre-flashing process and is determined based on both the detection output and the integrated value of reflection light quantity if the detection output is produced during the process of pre-flashing.
It is another object of the invention to provide a flash photography apparatus which operates in a first mode of automatically setting a flash photography aperture value by pre-flashing and in a second mode of permitting manual setting of the flash photography aperture value in the following manner: When the aperture value is manually set in the second mode, the aperture is controlled based on the aperture value determined in the first mode instead of the manually set aperture value, if the manually set value is smaller than the value determined by the first mode. Therefore, the aperture can be appropriately controlled even when the aperture value is inappositely set in the second mode.
An additional object of the invention is to provide a flash photography apparatus which permits synchronization of daylight flash photography by determining an aperture value based on the brightness when the brightness of an object to be photographed is greater than a predetermined value.